Eight Queens Problem

eight_queens.rb

class EightQueens
  def initialize
    @workset = (0...64).map { |i| [i] }
  end

  def queen_attacks(pos)
    (horizontal_attacks(pos) +
    vertical_attacks(pos) +
    main_diagonal_attacks(pos) +
    secondary_diagonal_attacks(pos)).uniq
  end

  def horizontal_attacks(pos)
    (0...8).map { |i| (pos / 8 * 8) + i }
  end

  def vertical_attacks(pos)
    (0...8).map { |i| (pos % 8) + i * 8 }
  end

  def main_diagonal_attacks(pos)
    (0...8).map {|i| i * 9 + (pos / 8 - pos % 8) * 8}.select {|i| i >= 0 && i < 64}
  end

  def secondary_diagonal_attacks(pos)
    (0...8).map {|i| 7 * (i + 1) + (pos / 8 - (8 - pos % 8) + 1) * 8}.select {|i| i >= 0 && i < 64}
  end

  def safe_squares(queens)
    squares = (0...64).to_a
    queens.each do |queen|
      squares -= (queen_attacks(queen) + [queen])
    end
    squares
  end

  def solve
    while @workset.length > 0
      queens = @workset.pop
      return queens if queens.length == 8
      safe_squares(queens).each do |square|
        @workset << (queens + [square])
      end
    end
  end
end

puts EightQueens.new.solve